Heegner's proof in 1952 that there is no tenth imaginary quadratic field of class number one is an interesting example. It was thought to be incorrect because of some gaps. Stark gave a correct proof in 1967 and explained how it was essentially the same as Heegner's proof. In 1969 Stark formally filled in the gap in Heegner's proof. Heegner "died before anyone really understood what he had done" (Goldfeld). This information comes from http://en.wikipedia.org/wiki/Stark-Heegner_theorem.